Valve guide stabilizing and sealing insert

ABSTRACT

An insert for sealing and stabilizing the valve stem of a poppet valve in the upper portion of a valve guide of an internal combustion engine. The insert comprises a cylindrical steel jacket having an inner layer of Teflon bonded thereto. The inner diameter of the insert corresponds to the diameter of the valve stem and extends coaxially and forms a substantial continuation of the valve guide wall. The insert extends beyond the upper end of the valve guide a distance of about 25 to 50 per cent of its length for stabilizing the reciprocation of the valve stem in cooperation with the remainder of the valve guide while preventing excessive amounts of lubricating oil from entering the valve guide.

United States Patent [1 1 Kammeraad [451 May 7,1974

[ VALVE GUIDE STABILIZING AND SEALING INSERT [75] Inventor: James A. Kammeraad, Holland,

Mich.

[22] Filed: Sept. 1, 1972 [21] Appl. No.: 285,607

[52] US. Cl 123/188 GC, 123/188 P, 308/5 V [51] Int. C1. F011 3/08 [58] Field of Search 123/188 P, 188 GC; 277/199; 308/238, DIG. 7, 5 V

[56] References Cited UNlTED STATES PATENTS 3,577,972 7/1970 Moray 123/188 P 1,972,650 9/1934 Gassaway et a1. 123/188 1,759,175 5/1930 Vaught 51/262 1,640,118 8/1927 Ford 123/90.67 3,168,320 2/1965 Sneed 308/238 3,033,623 5/1962 Thompson 308/238 3,022,685 2/1962 Armacost 308/238 X 2,965,040 12/1960 Eisenberg 308/240 X 2,813,524 12/1957 Brenneke 123/188 GC 908,604 1/1909 Odenkirchen.... 123/188 GC 897,023 8/1908 Schaap 123/188 P 314,079 3/1885 Thayer et a1 308/240 X FOREIGN PATENTS OR APPLICATlONS 571,554 8/1945 Great Britain 123/188 GC 198,944 6/1923 Great Britain 123/188 GC 388,333 8/1908 France 123/188 GC 334,854 1/1904 France 123/188 GC 1,148,812 7/1961 Germany 123/188 GC 604,551 11/1945 Great Britain 123/188 P 869,384 1/1960 Great Britain 123/188 GC Primary ExaminerCharles .1. Myhre Assistant ExaminerW. Rutledge, Jr.

Attorney, Agent, or Firm-Price, Heneveld, l-luizenga & Cooper [5 7] ABSTRACT An insert for sealing and stabilizing the valve stem of a poppet valve in the upper portion of a valve guide of an internal combustion engine. The insert comprises a cylindrical steel jacket having an inner layer of Teflon bonded thereto. The inner diameter of the insert corresponds to the diameter of the valve stem and extends coaxially and forms a substantial continuation of the valve guide wall. The insert extends beyond the upper end of the valve guide a distance of about 25 to 50 per cent of its length for stabilizing the reciprocation of the valve stem in cooperation with the remainder of the valve guide while preventing excessive amounts of lubricating oil from entering the valve guide.

19 Claims, 4 Drawing Figures VALVE GUIDE STABILIZING AND SEALING INSERT This invention relates to valve guide inserts for internal combustion engines and, more particularly, to a combination stabilizer and seal inserted within a portion of a valve guide of an internal combustion engine.

BACKGROUND OF THE INVENTION Valve guides of internal combustion engines through which the valve stem of a poppet valve reciprocates are susceptible to great amounts of wear over the lifetime of the engine. The wear in the guides is generally the greatest at either end thereof. Such wear is caused by the wobbling of the valve stem as it reciprocates through the valve guide with the center of the valve guide acting as a type of fiilcrum around which the stem pivots.

The latest internal combustion engines are designed, from an anti-pollution standpoint, such that the valves reciprocate through a greater distance than previously deemed necessary. Since the valves are typically actuated by a rocker-arm mechanism, which mechanisms have a longer arcuate path of travel and exert at least some lateral thrust on the valve stem, the increased travel distance has accentuated the wear problem. Further, the use of non-leaded fuels has tended to reduce the lubrication of the valve guide thereby also increasing the overall wear thereof.

It is customary, in the internal combustion engine art, to provide a seal at each of the valve stem locations to prevent the leakage of undesirable quantities of oil into the combustion chamber through the valve guide passageway. These seals typically are of the umbrella type which reciprocates with the valve stem or of the positive" type which is affixed in stationary fashion to the exposed shoulder portion of the valve guide. Satisfactory seals for this purpose are available although the same do increase the cost of engine manufacture and maintenance. One of the objects of this invention, as will be apparent hereinafter, is to provide an insert of the general type described which functions not only to stabilize the wobbling tendency of the valve stem and, thus, reduce guide wear, but also to perform the dual function of the seal, rendering a separate seal element unnecessary.

SUMMARY OF THE INVENTION Accordingly, this invention provides a combination stabilizer and seal for valve guides having the dual primary purposes of preventing the wear of the ends of valve guides and facilitating the lubrication of the valve guide. An accompanying purpose of the invention is to provide an oil seal which prevents excessive lubricating oil from the upper valve assemblies from entering the valve guide, and, thus, the combustion chamber of the cylinder therebelow.

The valve guide stabilizing seal of the present invention generally comprises a cylindrical insert inserted in a valve guide in an internal combustion engine such that it extends approximately to 50 per cent of its length beyond the end of the extending shoulder portion of the valve guide at the upper end thereof. The insert comprises an outer layer or jacket of metal such as steel and an inner concentric layer of a lubricious plastic bearing material which, in the'preferred embodiment, is a layer of Teflon bonded securely to the outer metal layer. The cylindrical insert includes an axial slot therethrough and extending along one side thereof in a stepped fashion for allowing the bushing to be compressed to the size of an enlarged opening formed in the upper end of the valve guide. The stepped configuration of the axial slot prevents the opposing edges along the slot from moving axially relative to one another during installation of the insert as well as retarding oil leakage through the slot.

When force-fitted in the enlarged portion of the valve guide, the inner diameter of the stabilizing seal will form a substantial continuation of the valve guide. The ends of the stabilizing and sealing insert may be chamferred to facilitate the insertion in the enlarged area and to prevent a build-up of lubricating oil thereon. Further, the Teflon inner lining may include knurling or spiral groove to aid the controlled lubrication of the valve stem.

These and other objects, advantages and features of the invention will become apparent from a study of the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a fragmentary, cross-sectional, side elevation of a typical poppet valve mounted in a valve guide of an internal combustion engine cylindrical head, the valve guide including the stabilizing seal of the present invention;

FIG. 2 is a side elevation of the stabilizing seal shown in FIG. 1;

FIG. 3 is a cross-sectional view of the stabilizing seal taken along plane Ill-lll of FIG. 2; and

FIG. 4 is a fragmentary, enlarged, cross-sectional view of the stabilizing seal of FIGS. 1 through 3 shown during its insertion within the valve guide of a typical cylinder head.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to-FIG. l, the valve guide stabilizing seal 10 of the present invention is shown inserted in the upper end 13 of a typical valve guide 12 formed in a typical cylinder head 14 of an internal combustion engine. A typical reciprocating poppet valve 16, including a valve head 18 and a valve stem 20, is reciprocally mounted within the combined valve guide 12 and stabilizing seal 10. One or more valve springs 22 and 24 are mounted concentrically over an extending shoulder portion 26 of valve guide 12 and engage a valve retainer (not shown) secured to the upper end of the valve stem 20 (not shown) in conventional fashion. The inside diameter of the shoulder portion 26 defines the upper portion 13 of the valve guide. An enlarged or relieved area 27 is formed in upper portion 13 to receive the stabilizing seal bushing therein. The valve 16 is reciprocated in the conventional manner by means of either a push rod and rocker-arm assembly (not shown) or an overhead camshaft (not shown), either of which engages the upper end of valve stem 20.

The stabilizing seal 10 is mounted as shown in FIG. 1 to provide an extension of the normal length of the guide valve 12 as Well as an effective seal preventing excessive amounts of lubricating oil from the rocker arm housing from entering the valve guide while simultaneously facilitating the lubrication of the valve stem '20 by means of the Teflon inner layer, as will be more fully described below. i

As shown in FIGS. 2 and 3, the valve guide stabilizing seal includes inner and outer concentric layers 30 and 32, respectively, which layers are securely bonded together to form the walls or sides of the insert 10. The insert 10 includes an axially extending slot or cut 34 along one side thereof. The cut 34 provides a gap along the length of the insert when it is in its related condition prior to insertion into the relieved area of the valve guide. This gap allows the insert to radially compress when pressed or force-fitted into the relieved area 27 during installation. The relative dimensions of relieved area 27, the insert 10 and the valve stem are such that (l) the insert is tightly received and retained by area 27; (2) the slot 40 closes tightly during such installation, the insert then assuming a circular cross-sectional configuration; and (3) the inner diameter of the installed insert equals as closely as possible the diameter of the valve stem.

As shown in FIG. 2, the axially extending slot 34 is formed by two parallel axially extending slot portions 36 and 38 which are spaced circumferentially of one another and joined by a circumferentially extending cut 40 therebetween. The cut 40 forms a step in the axially extending cut 34, which step prevents the opposing edges 42.and 44 along the length of the slot 34 from moving axially relative to one another as the bushing 10 is inserted in the relieved area 27 in the upper portion 13 of valve guide 12 and is concentrically compressed thereby. Step 40 also functions to retard undesirable oil flow along the slot 34 during operation of the engine.

Insert 10 includes chamfers 46 and 47 on either end I thereof. The chamfer on the end inserted in relieved area 27 facilitates that insertion while the chamfer on the remaining end provides a sloped area which prevents the build-up of lubricating oil adjacent the point at which valve stem 20 enters the insert 10 (FIGS. 2 and 4).

The outer layer 32 of the stabilizing and sealing insert 10 is formed from a mild steel having a thickness of between approximately 0.0l0 and 0.019 inches. The inner concentric layer 30 of bushing 10 is preferably formed from a lubricious, polymeric resin compound such as polytetrafluoroethylene (Teflon). The Teflon layer-30 is securely bonded to the outer steel layer 32 by conventionally known methods. The Teflon layer is in the range of approximately 0.006 to 0.020 inches. The overall thickness of the insert sidewall is therefore between 0.016 and 0.039 inches. Various thicknesses of the steel and Teflon layers may be used although, generally, in the preferred embodiment, the thickness of the inner Teflon layer 30 will be equal to or less than the outer steel layer 32. It has been found that with somewhat increased thicknesses of the layers 30 and 32 within the ranges defined above, the support for valve stem 20 in valve guide 12 is significantly increased, thereby further reducing the wobbling tendency of the valve stem during its reciprocation as well as the wear accompanying such wobbling.

Although Teflon is preferred for the inner layer 30, other self-lubricating or lubricious plastic resins capable of withstanding high temperatures and long periods of wear, as is Teflon, may be used as a substitute for the Teflon therein.

As shown in FIG. 1, when the stabilizing seal 10 is installed in the upper portion of the valve guide, it extends a predetermined distance Y above the end of the extending shoulder portion 26 of the valve guide. The amount Y of protrusion or projection of the stabilizing seal 10 is designed to be, in the preferred embodiment, within the range of 25 to 50 per cent of the overall length X of the cylindrical bushing 10 forming the stabilizing seal. It has been found that when the protrusion of the seal is maintained within this range, the stability of the reciprocating valve stem 20 is greatly increased thereby reducing the wear on the upper and lower ends of the valve guide and providing longer life for the engine. Further, the need for mounting additional oil seals over the valve springs 22 and 24 or on the upper portion of the valve stem 20, to prevent excessive amounts of lubricating oils splashed over the upper valve assemblies from entering and draining through the valve guide 12 into the combustion chamber, is obviated by the sealing function of the bushing 10.

In this connection, the seal 10 will have an inside diameter corresponding to the outside diameter of the valve stem 20, i.e., the tolerance between the stabilizing and sealing bushing and valve stem will be as close to zero as possible. The valve guide below the seal 10 may have standard production tolerances normally encountered in engine manufacturing.

Referring now to FIG. 4, the method for installing the stabilizing seal 10 in the enlargedor relieved area 27 of upper portion 13 of valve guide 12 is shown in greater detail. First, the upper end of the valve guide is bored with appropriate boring tools (not shown) to form the enlarged portion 27 having a diameter larger than that of the valve guide and equivalent to the outside diameter of the compressed insert 10. Then, as depicted in FIG. 4, a punch-like tool 50 is positioned at the upper end of the stabilizing seal 10 and is struck to force the seal 10 through a mounting adapter 52 and into the relieved area 27 of valve guide 12. The adapter 52 includes a chamfer 53 on its inner diameter to facilitate the starting of the seal 10 downwardly therethrough. Otherwise, the inner diameter of the adapter 52 is equivalent to the outside diameter of the compressed stabilizing and sealing insert.

The tool 50 includes a shoulder 24 formed by a reduced diameter section 55 on the end of the tool to assist positive engagement of the tool with the insert 10 during the installation process. The seal 10 is forced downwardly in a press-fit engagement into the relieved area 27 and into abutment with shoulder 29 at the lower end thereof. Use of the adapter 52 automatically compresses the seal 10 to the diameter of enlarged area 27 such that insertion is greatly facilitated. Additionally, the inside diameter of the upper end of extending shoulder portion 26 may include a chamfer 28 as shown in FIG. 4 to'facilitate the installation of the seal 10. Similarly, the insert 10 may include a chamfer 46 to aid the insertion thereof on one end as shown in FIGS. 2 and 4. As mentioned above, chamfer 47 on the opposite end prevents a build-up of oil on the upper end of insert 10 thereby further preventing oil from entering the valve guide passageway. After insertion of the stabilizing and sealing insert 10 in enlarged portion 27 of shoulder 26, the inner layer 30 of Teflon may be knurled or have a spiraling groove formed therein as at 31 with a knurling spiraling tool. Alternatively, the knurling or spiraling groove may be formed prior to the forming of the insert 10 into its cylindrical shape.

As shown in FIGS. 1 and 4, the compressed insert has an outer diameter corresponding in size to the enlarged area 27 in shoulder 26 which diameter is larger than the inside diameter of valve guide 12. When installed in the relieved area 27, the inside diameter of the insert 10 forms a substantial continuation of the valve guide 12 thereby forming a coaxial extension of .tion is performed at the interface of the upper extremity of Teflon layer and the valve stem. The stabilizing seal 10 eliminates wobble during reciprocation of the valve stem especially in recently developed engines having longer valve reciprocation distances. The stabilization of the valve stem thereby significantly reduces the amount of wear in both the upper and lower portions of valve guide 12.

Additionally, the stabilizing seal of the present invention provides a lubricious inner layer of Teflon which further reduces the wear on the valve stem during the stabilizing support thereof. The combined effect of both the stabilization and sealing, as well as the selflubricating properties of the inner Teflon layer, produce significantly reduced wear of the valve guide and valve stem while lengthening the life of the internal combustion engine in which they are installed.

It will be understood that terms such as upper and lower" as used in this description are used for explanation purposes only and are not intended to limit the scope-of the invention.

While one form of the invention has been shown and described, other forms will now be apparent to those skilled in the art. Therefore, it will be understood that the embodiments shown in the drawings and described above is merely for illustrative purposes and is not intended to limit the scope of the invention which is defined by the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

l. A valve guide stabilizing seal for internal combustion engines comprising an insert mounted within the upper portion of a valve guide, said insert comprising: a cylinder having an outer layer of metal and a bonded inner layer of a lubricious, plastic bearing material; said cylinder including an axial slot extending substantially along the entire length thereof allowing said cylinder to be compressed; said cylinder, when compressed, having an inside diameter corresponding to the outside diameter of a valve stem reciprocally mounted within said valve guide thereby sealing said valve stem, and an outside diameter greater than said inside diameter of said valve guide and corresponding to the diameter of a relieved area in said upper portion of said valve guide; said insert extending upwardly beyond the upper boundary of said valve guide, the relative dimensions of said relieved area and said insert being such that said slot will close tightly when said insert is force-fitted into and retained independently by said relieved area.

2. The stabilizing seal of claim 1 wherein said cylinder has two ends at least one of which includes a chamfer on the outside diameter thereof.

3. The stabilizing seal of claim 2 wherein both end of said cylinder include a chamfer.

4. The stabilizing seal of claim 1 wherein said plastic bearing layer has a thickness less than that of said metallic layer.

5. The stabilizing seal of claim 4 wherein said plastic bearing layer is Teflon.

6. The stabilizing seal of claim 1 wherein said plastic bearing layer is Teflon.

7. The stabilizing seal of claim 1 wherein said insert extends upwardly beyond the upper boundary of said I valve guide a distance between about 25 to 50 per cent of the overall length of said insert.

8. The stabilizing seal of claim 1 wherein said plastic bearing layer has a spiral groove for further lubrication of said valve stem.

9. The stabilizing seal of claim 1 wherein said axial slot includes two parallel portions spaced circumferentially apart and joined by a circumferential portion, said slot adapted to allow said insert to compress to the size of said enlarged area upon the insertion of said bushing in said upper portion of said valve guide.

10. In an internal combustion engine including a cylinder head, at least one valve guide having an extending shoulder portion including a portion of said valve guide, and at least one valve, including a valve stem, reciprocally mounted within said valve guide; the combination of said valve guide and means for sealing and stabilizing said valve stem in said valve guide; said means including a cylindrical insert having an outer layer of metal and an inner layer of a lubricious, synthetic, resinous bearing material; said insert mounted in the upper portion of said valve guide within said shoulder and extending beyond the end of said shoulder for stabilizing said valve stem and sealing the same to prevent undesired oil passage through said guide; said insert having an inner diameter corresponding to the diameter of said valve stem; said inner diameter forming a substantial continuation of said valve guide; said insert being retained within said valve guide independently of any mechanism for operating the valve which includes said valve stem and further including an axial slot through both of said metallic and plastic layers extending substantially along the entire length of said insert.

11. The combination of claim 10 wherein said lubricious, synthetic bearing layer comprises a layer of Tefion.

12. The combination of claim 11 wherein said metallic layer is thicker than said Teflon layer.

13. The combination of claim 10 wherein said cylindrical insert includes two ends at least one of which includes a chamfer on the outside diameter thereof.

14. The combination of claim 13 wherein said end of said valve guide at said extending shoulder includes a chamfer to facilitate installation of said insert.

15. The combination of claim 10 wherein said insert extends beyond the end of said shoulder about 25 to 50 per cent of the length of said insert.

17. The combination of claim 10 wherein said axial slot includes two circumferentially spaced portions each extending approximately one-half the axial length of said insert such that the ends thereof are adjacent one another; said adjacent ends of said portions being joined by a partially annular slot.

18. The combination of claim 10 wherein said axial slot includes two parallel portions spaced circumferentially apart and joined by a circumferential portion at the middle of said insert; said slot adapted to allow said insert to compress to the size of said valve stem when inserted in said upper portion of said valve guide.

19. A combination stabilizing and sealing insert for use in the valve guide of an internal combustion engine, said valve guide having a valve stem reciprocally mounted therein, the extremity of said guide having a relieved area therein to accept and retain a section of said insert, said insert comprising a cylindrical metal jacket having a layer of lubricious plastic bearing material bonded to the interior thereof, said jacket and plastic layer having a longitudinal slot therethrough along along the entire length of said insert when said insert is in a relaxed condition prior to insertion into said relieved area, said gap being closable by radial compression upon insertion of said insert into said relieved area, the relative dimensions of said relieved area, the insert and the valve stem being such that (a) the insert will be tightly received and retained by said relieved area independently of any mechanism for operating the valve which includes said valve stem upon insertion of the insert thereinto; (b) said gap will close tightly upon such insertion; and (c) the inner diameter of the installed insert equals as closely as possible the outer diameter of the valve stem; the axial length of said relieved area being substantially less than the axial length of said insert whereby a substantial portion of said insert will extend in exposed fashion beyond said guide. 

1. A valve guide stabilizing seal for internal combustion engines comprising an insert mounted within the upper portion of a valve guide, said insert comprising: a cylinder having an outer layer of metal and a bonded inner layer of a lubricious, plastic bearing material; said cylinder including an axial slot extending substantially along the entire length thereof allowing said cylinder to be compressed; said cylinder, when compressed, having an inside diameter corresponding to the outside diameter of a valve stem reciprocally mounted within said valve guide thereby sealing said valve stem, and an outside diameter greater than said inside diameter of said valve guide and corresponding to the diameter of a relieved area in said upper portion of said valve guide; said insert extending upwardly beyond the upper boundary of said valve guide, the relative dimensions of said relieved area and said insert being such that said slot will close tightly when said insert is force-fitted into and retained independently by said relieved area.
 2. The stabilizing seal of claim 1 wherein said cylinder has two ends at least one of which includes a chamfer on the outside diameter thereof.
 3. The stabilizing seal of claim 2 wherein both end of said cylinder include a chamfer.
 4. The stabilizing seal of claim 1 wherein said plastic bearing layer has a thickness less than that of said metallic layer.
 5. The stabilizing seal of claim 4 wherein said plastic bearing layer is Teflon.
 6. The stabilizing seal of claim 1 wherein said plastic bearing layer is Teflon.
 7. The stabilizing seal of claim 1 wherein said insert extends upwardly beyond the upper boundary of said valve guide a distance between about 25 to 50 per cent of the overall length of said insert.
 8. The stabilizing seal of claim 1 wherein said plastic bearing layer has a spiral groove for further lubrication of said valve stem.
 9. The stabilizing seal of claim 1 wherein said axial slot includes two parallel portions spaced circumferentially apart and joined by a circumferential portion, said slot adapted to allow said insert to compress to the size of said enlarged area upon the insertion of said bushing in said upper portion of said valve guide.
 10. In an internal combustion engine including a cylinder head, at least one valve guide having an extending shoulder portion including a portion of said valve guide, and at least one valve, including a valve stem, reciprocally mounted within said valve guide; the combination of said valve guide and means for sealing and stabilizing said valve stem in said valve guide; said means including a cylindrical insert having an outer layer of metal and an inner layer of a lubricious, synthetic, resinous bearing material; said insert mounted in the upper portion of said valve guide within said shoulder and extending beyond the end of said shoulder for stabilizing said valve stem and sealing the same to prevent undesired oil passage through said guide; said insert having an inner diameter corresponding to the diameter of said valve stem; said inner diameter forming a substantial continuation of said valve guide; said insert being retained within said valve guide independently of any mechanism for operating the valve which includes said valve stem and further including an axial slot through both of said metallic and plastic layers extending substantially along the entire length of said insert.
 11. The combination of claim 10 wherein said lubricious, synthetic bearing layer comprises a layer of Teflon.
 12. The combination of claim 11 wherein said metallic layer is thicker than said Teflon layer.
 13. The combination of claim 10 wherein said cylindrical insert includes two ends at least one of which includes a chamfer on the outside diameter thereof.
 14. The combination of claim 13 wherein said end of said valve guide at said extending shoulder includes a chamfer to facilitate installation of said insert.
 15. The combination of claim 10 wherein said insert extends beyond the end of said shoulder about 25 to 50 per cent of the length of said insert.
 16. The combination of claim 10 wherein said lubricious, synthetic bearing layer includes spiral grooving therealong for additional lubrication of said valve stem.
 17. The combination of claim 10 wherein said axial slot includes two circumferentially spaced portions each extending approximately one-half the axial length of said insert such that the ends thereof are adjacent one another; said adjacent ends of said portions being joined by a partially annular slot.
 18. The combination of claim 10 wherein said axial slot includes two parallel portions spaced circumferentially apart and joined by a circumferential portion at the middle of said insert; said slot adapted to allow said insert to compress to the size of said valve stem when inserted in said upper portion of said valve guide.
 19. A combination stabilizing and sealing insert for use in the valve guide of an internal combustion engine, said valve guide having a valve stem reciprocally mounted therein, the extremity of said guide having a relieved area therein to accept and retain a section of said insert, said insert comprising a cylindrical metal jacket having a layer of lubricious plastic bearing material bonded to the interior thereof, said jacket and plastic layer having a longitudinal slot therethrough along the entire length of said insert, said slot forming a gap along the entire length of said insert when said insert is in a relaxed condition prior to insertion into said relieved area, said gap being closable by radial compression upon insertion of said insert into said relieved area, the relative dimensions of said relieved area, the insert and the valve stem being such that (a) the insert will be tightly received and retained by said relieved area independently of any mechanism for operating the valve which includes said valve stem upon insertion of the insert thereinto; (b) said gap will close tightly upon such insertion; and (c) the inner diameter of the installed insert equals as closely as possible the outer diameter of the valve stem; the axial length of said relieved area being substantially less than the axial length of said insert whereby a substantial portion of said insert will extend in exposed fashion beyond said guide. 